Electro-mechanical switches are utilized in a variety of industrial, consumer and commercial applications. Certain types of electrical switching applications require a mechanical switch that can operate properly with a slowly-applied, low-actuation force. Such a switch must also be extremely reliable and generate an accurate, repeatable response, while possessing a small actuation differential and/or low energy requirement. These requirements arise perhaps most commonly in applications involving electro-mechanical thermostats, which are utilized for controlling heating and cooling in homes and buildings where coils of standard bimetal strips form the switch actuation elements. For many years this thermostatic switching function has been performed by mercury bulb switch elements.
Due to the environmental concerns associated with the use of mercury, it is anticipated that electro-mechanical switches will eventually replace mercury-based switches. Legislation currently being drafted and passed in a variety of countries, including the United States, is aimed at banning the use of mercury in most consumer-based applications. Thus, non-mercury based switches must be developed to replace such mercury-type switching mechanisms.
Some attempts have been made at replacing mercury-switching devices. For example, so-called “snap action” switches have been designed to address the environmental concerns that mercury bulb switch elements raise. As utilized herein, the term “snap action switch” generally refers to a low actuation force switch, which utilizes an internal mechanism to rapidly shift or snap the movable contact from one position to another, thus making or breaking electrical conduction between the movable contact and a fixed contact in response to moving an operating element of the switch, such as a plunger, a lever, a spring, or the like from a first to a second operating position. Typically, these switches require only a few millimeters of movement by the operating element to change the conduction state of the switch.
Such switches can safely and reliably operate at a current level of several amperes using the standard 24 VAC power that thermostats control. However, when actuated by a slowly-applied, low-actuation force such as is provided by a thermostat's coiled bimetal strip, snap action switches may occasionally hang in a state between the two conducting states, or may switch so slowly between the two conducting states that unacceptable arcing and/or increased temperature can occur when entering the non-conducting state. Either condition gives rise to unacceptable reliability and predictability of operation. Furthermore, these switches frequently have unacceptably large differentials, which means that the position of the operating element at which actuation of the switch to one state occurs differs substantially from the position of the actuation element at which actuation of the switch to the other state occurs. If the differential is too large, then the temperature range that the controlled space experiences is also too large.
Thermostats with electronic components are generally known in the art. An example of an electro-mechanical thermostat that has been utilized in commercial, consumer and industrial applications is the T87 thermostat produced by Honeywell International, Inc. (“Honeywell”) of Minneapolis, Minn. An example of the T87 thermostat is disclosed in the publication “Thermostats T87F,” Form Number 60-2222-2, S.M. Rev. 4–86, which is incorporated herein by reference. Another example of the T87F thermostat is disclosed in the publication “T87F Universal Thermostat,” Form Number 60-0830-3, S.M. Rev. 8–93, which is also incorporated herein by reference. The T87F thermostat, in particular, provides temperature control for residential heating, cooling or heating-cooling systems. U.S. Pat. No. 5,262,752, which is incorporated by reference, is an example of an electrical switch assembly that forms the temperature responsive element in a thermostat.
One of the problems encountered in the efficient utilization of many thermostats in use today is the problem of actuating an electro-mechanical switch with a slow-moving actuator, such as a bimetal element, without sacrificing the switch's electrical life. For example, electro-mechanical thermostats, such as the T87 line of thermostats manufactured by Honeywell, utilize a bimetal element as the temperature-sensing device. In the operation of the thermostat, the bimetal element moves a small amount at a slow rate. Actuating a switch directly off the bimetal element results in an inordinate amount of time spent, during the switching cycle, at or near snap-over. Electro-mechanical switches have low contact forces near snap-over and zero contact forces at snap-over. When the switch contact forces are low or zero, the amount of electrical resistance at the contact interface increases. As the electrical resistance to current passing through the switch increases, the heat also increases. The electrical life of an electro-mechanical switch is reduced with time as the current is carried at or near the snap-over points.
The present inventors have thus concluded, based on the foregoing, that a need exists for an improved apparatus, including a method thereof, for effectively actuating an electro-mechanical switch.